The upstream path of the current Cable Television Data-Over-Cable Service Interface Specification (CATV DOCSIS) or Open Cable Application Protocol (OCAP) communication scheme can be impaired for various reasons. FIG. 1 illustrates a conventional cable TV network that has DOCSIS or OCAP internet access. A Cable Modem Termination System 1 (CMTS) has three connections: 1) a network interface to servers 17 via the internet 16; 2) downstream control and data delivery via a signal combiner 2; and 3) upstream control and data reception via a signal splitter 3. FIG. 1 illustrates one “Node” of a CATV plant, which serves up to 200 homes with DOCSIS or OCAP service. Often a “Hub” will have over one hundred Nodes and a city the size of Indianapolis can have five or six hub sites.
The downstream signal of the CMTS 1 is coupled with the other signals, e.g. TV, Test, Telephony or others, via the Signal Combiner 2. The output of the Signal Combiner 2 is connected to a Fiber Transceiver Node 4, which converts the combined downstream signal suite from RF signals to optical signals, and delivers the combined signal suite to a remote location via a fiber optic link 5. The downstream optical signals are converted back to RF signals at a Transceiver Node 6, which is also optically coupled to the optical link 5. The RF signals from the transceiver node 6 are delivered to different residences via a cable Distribution Network 7. Certain residences 8, 9 and 10 use cable modems CM1, CM2 and CM3, respectively. A control signal from the CMTS 1 is detected and demodulated by each of the cable modems CM1, CM2 and CM3. When a cable modem, e.g. CM1, CM2 or CM3, wants to talk back to the CMTS 1, the cable modem will wait until the CMTS 1 indicates that the upstream path is free, the cable modem then requests a time slot from the CMTS 1 via an upstream signal. The upstream signal is coupled to the distribution network 7, and then converted to an optical signal at the fiber transceiver 6. The upstream optical signal travels back to the hub site or head-end via optical cable link 15, where the fiber transceiver 4 optically connected to the optical cable link 15, converts the upstream optical signal back to an RF signal. The upstream RF signal is then routed to the signal splitter 3, where a portion of the upstream signal is sent to the CMTS 1. The CMTS 1 thus becomes the link between the cable modems CM1, CM2 and CM3 and the servers 17 via the internet 16.
Time division multiplexed access (TDMA) is the preferred communication link, since each cable modem CM1, CM2 and CM3 is randomly picking a time to request upstream signal transmissions, and packet sizes from the cable modems CM1, CM2 and CM3 vary. Various other systems and features can be applied which extend the behavior and improve noise immunity, but essentially TDMA is used. The cable modem transmissions, i.e. bursts, are at random intervals from an outsiders point of view.
Referring to FIGS. 1 and 2, each cable modem CM1, CM2 and CM3 will transmit a packet 19 or series of packets 11, 12 and 13, respectively, in either a blank time slot identified by the CMTS 1 or into a scheduled timeslot assigned by the CMTS 1. The distribution network 7 combines all of the CM packet series 11, 12 and 13 together into a single upstream signal 14, whereby all of the individual series are indistinguishable from an outside observer. OCAP operates similarly to the DOCSIS protocol and engineers familiar with the state of the art would see the commonalities and differences, in particular OCAP is used for set-top box command and control.
A problem exists for upstream path monitoring in the DOCSIS 3.0. protocol, because DOCSIS is improving the upstream path bandwidth, building in the capacity for quadrupling or more the amount of data that will be sent back to the CMTS 1 by using four data channels instead of the one or two channels currently in use. Accordingly, the CATV upstream path is becoming more crowded, which presents a problem for return path monitoring. The spectrum which was formerly empty now contains signals, and looking for noise is more difficult; accordingly, better ways are needed to prove that the CATV upstream path is performing optimally. New monitoring systems must be able to monitor in a crowded or full upstream spectrum.
The conventional method of monitoring consists of setting thresholds for specific frequencies from a spectrum scan. If the signal crosses these thresholds, it is assumed that there is a problem on the network; unfortunately, this can lead to false prioritization for node repair. Monitoring MER is one method of gauging active channel performance without counting on empty spectrum. Since bad MER is the equivalent of losing data, and the multiple service operator's (MSO) revenues are based on both modem data traffic and set-top box data traffic, setting alarms based on these raw numbers is a more effective way of prioritizing nodes to work on. Combine this with the current monitoring statistics, and the user will find out when they can schedule field maintenance with a metric that has a direct impact on revenue.
Currently there is hardware, Filtronics model ST-260C DOCSIS 2.0 Protocol Analyzer1, that monitors the upstream signal 14, and can determine which cable modem is transmitting packets; however, in monitoring or remote applications attaching a monitoring device to every CMTS downstream thereof would be cost prohibitive. United States Patent Publication No. 2005/0047442 published in the name of Volpe et al. on Mar. 3, 2005, describes a system in which a MAC address associated with a specific cable modem can be entered, and the signal quality of the upstream channel from the specific cable modem can be determined.
The distribution network, illustrated in FIG. 1, has a number of possible impairments that can affect the CM upstream signal 14. Some of the impairments affect every cable modem, whereas other impairments affect only a subset of modems. Another object of the present invention is to find the impairments that affect only a subset of the cable modems.